1. Field of the Invention
The invention relates to a control apparatus for a four wheel drive vehicle.
2. Description of the Related Art
Four wheel drive vehicles, in which a driving force from a drive source (engine) is always transmitted to a pair of right and left main driving wheels (front wheels), and is transmitted to a pair of right and left auxiliary driving wheels (rear wheels) via a plurality of clutches, have been known (refer to Japanese Patent Application Publication No. 2010-254058 (JP 2010-254058 A) and Japanese Patent Application Publication No. 2012-61923 (JP 2012-61923 A)).
In each of the four wheel drive vehicles described in JP 2010-254058 A and JP 2012-61923 A, a dog clutch is disposed between the drive source and a propeller shaft that transmits the driving force in the longitudinal direction of the vehicle. Further, a multiple disc clutch is disposed between an auxiliary driving wheel-side differential apparatus to which the driving force is transmitted by the propeller shaft and one of the auxiliary driving wheels. When the vehicle travels in a two wheel drive mode in which the driving force is transmitted only to the main driving wheels, the transmission of the driving force via the dog clutch and the multiple disc clutch is interrupted, and therefore, rotation of the propeller shaft and rotation of a differential case of the auxiliary driving wheel-side differential apparatus are restrained to reduce the travel resistance.
When the vehicle travels in a four wheel drive mode in which the driving force of the drive source is transmitted to the main driving wheels and the auxiliary driving wheels, the dog clutch is engaged, and therefore, a driving force in accordance with the transmission torque of the multiple disc clutch is transmitted to the auxiliary driving wheel side. The transmission torque of the multiple disc clutch is controlled by an Electronic Control Unit (ECU). The ECU computes a driving force that needs to be transmitted to the auxiliary driving wheel side, on the basis of signals from various sensors that detect the traveling state of the vehicle, and then controls the transmission torque of the multiple disc clutch according to a result of the computation.
The inventor of the present application measured the driving force transmitted to the auxiliary driving wheel side in a four wheel drive vehicle configured as described above, and found that the driving force transmitted to the auxiliary driving wheel side when the vehicle turns left is different from that when the vehicle turns right, even if the transmission torque of the multiple disc clutch is controlled in the same manner. The inventor of the present application earnestly investigated the cause of this phenomenon, and obtained a finding that the difference in the driving force transmitted to the auxiliary driving wheels depending on the direction of turn of the vehicle is related to the Torque Bias Ratio (TBR) of the differential apparatus at the auxiliary driving wheel side, and made the invention of the present application.